"I'm absolutely devastated by this news," says Moore, 76, who herself underwent surgery in 2011 for a benign tumor in the protective membrane around the brain – the same spot where Harper's malignant cancer cells are located.

"Valerie has given so much joy, laughter and love to the world," says Moore. "I join her fans and send much love and positive thoughts to her and her family during this difficult time."

One day before going public with her diagnosis in PEOPLE, Harper called her costar from the show Mary Tyler Moore.

As costars in the early 1970s, "she was kind and generous and loved to give gifts," recalls Harper, who played acerbic New Yorker Rhoda Morgenstern to Moore's sweet-natured Midwesterner Mary Richards. "Our first Christmas she gave me a big long key made into a necklace. It had the address of our made up apartment, 119 Weatherly. I learned so much from her and she shared so much."

And Moore continues to share support and advice with her dear friend.

Before ending their phone call, "Mary said, 'One day at a time,'" says Harper. "That's how we've got to live our lives. That's the best we can do."